Time Division Multiple Access communications systems are widely used in the communications field because of their efficient use of available bandwidth. The key to TDMA operation is the necessity for each of the transmitting stations to transmit their respective burst transmissions in an appropriate time sequence so that the bursts appear sequentially at a repeater or common location in the system. Misalignment of the timing results in bursts which overlap to a lesser or greater extent. Overlap caused by improper timing can degrade the execution of the communication function. Early TDMA systems used a reference station, commonly identified to all of the stations in the network. A specified transmission burst of the reference station (whether or not it was a special reference burst), when received by the other stations, would be used for timing purposes. Because of the essential nature of the reference, in enabling the other stations to time their bursts, elaborate procedures were worked out in order to handle the condition in which the burst from the reference station was absent. (See, for example, U.S. Pat. No. 4,054,753 and the references cited therein.)
U.S. Pat. Nos. 4,262,356 and 4,630,267 appear to show how burst timing for one station can be implemented without a frame reference by merely timing from another burst in the frame. (U.S. Pat. No. 4,574,379 has some similarity in this regard.) The problem with this arrangement is that it does not take account of the timing of more than a single burst and as a result will insure the propagation of any timing error in the burst on which the timing is based. While both U.S. Pat. Nos. 4,262,356 and 4,630,267 mention timing based on all received bursts, the transmitting station's burst is based on the earliest transmit time indication of all the other bursts, see col. 4, lines 24-27 of U.S. Pat. No. 4,630,267 as well as col. 6, lines 7-14 and col. 9, lines 25-31 of U.S. Pat. No. 4,262,356.
Over and above the reference timing function, there is another synchronization requirement in TDMA networks which are carrier based. The synchronization function requires that each of the stations have a common estimate of the transmit carrier frequency.
In systems which use a reference station, each of the other stations can attempt to synchronize their own transmit carrier to their estimate of the reference station's transmit carrier frequency. In a system, such as to be described hereinafter, where there is no reference station, clearly some other procedure must be used so as to provide a station with information respecting an appropriate transmit carrier frequency. U.S. Pat. No. 4,489,413 proposes a system for synchronizing transmit carrier frequencies in a TDMA/TDM system. However, in the system described in U.S. Pat. No. 4,489,413, the stations receive a TDM (not a TDMA) signal from a central source. In contrast to U.S. Pat. No. 4,489,413, a typical TDMA system directs a frame of bursts from a geosynchronous satellite to each of the stations. Each burst in the frame may have originated from a different station and while each of those stations is nominally operating on an identical transmit carrier frequency, account must be taken of the fact that the frequencies will, of necessity, differ. This variation in transmit carrier frequencies is maintained inasmuch as the satellite transponder merely translates each of the bursts it receives by a fixed frequency offset. As a result, any differences between transmit carrier frequencies at the different transmitting stations is reflected in the carrier frequency of the bursts received by the stations. Thus the arrangement described in U.S. Pat. No. 4,489,413 does not resolve the problem of identifying an appropriate carrier frequency in a TDMA system.